Storage devices, such as disc drives, typically use servo systems to position a read or write head over a recording track where digital information is stored. Like all mechanical structures, the servo system has resonant frequencies at which it physically resonates. When the servo system resonates it can become unstable or unpredictable if the resonant frequencies fall within range of operating frequencies of the servo system, which is the range of frequencies at which the servo system can move a head back and forth across the disc.
During design and/or manufacturer of current storage devices, the gain of the servo loop system is measured to determine resonant frequencies of the servo loop and the relative magnitudes of the servo loop gain at those frequencies. Typically, the servo loop gain's magnitude will have localized peaks near resonant frequencies. The magnitude of these peaks, as well as their relationship to the phase response of the servo loop, determine whether the servo loop is unstable.
Currently servo loop gain determinations are made by positioning the head at a radial position along a track and introducing noise signals of different frequencies into the servo loop system. The response of the servo loop at those frequencies is then measured to determine the open loop gain and phase response.
Unfortunately, such measurement systems are less than ideal and do not accurately describe the servo loop gain distribution of a disc drive during normal operations.
The present invention addresses these and other problems, and offers other advantages over the prior art.